Production of polyethylene

ABSTRACT

Polyethylene is produced by polymerization of ethylene alone or with comonomers or telogens (modifiers) in an elongated tubular reactor having an inlet and outlet and at least one reaction zone and one cooling zone in the presence of free radical or free oxygen yielding initiator at elevated temperatures and pressures by passing the reaction mixture through each of the reaction zones of the tubular reactor having internal diameters between about 0.5 and 3 inches at bulk fluid velocities sufficient so that the Flow Number in each reaction zone is greater than 3.3 ft. 2  /sec. Flow Number is defined as the bulk fluid velocity in ft./sec. times diameter in feet. With Flow Numbers in excess of 3.3 ft. 2  /sec. in the reaction zones, it has been found that the effective reaction volume in the reaction zone has been increased and, accordingly, a more efficient process for producing a high quality polyethylene.

This is a continuation of application Ser. No. 625,415, filed 10/24/75,and now abandoned, which is a continuation of Ser. No. 386,915, filedAug. 9, 1973, and now abandoned, which is a continuation of Ser. No.126,300, filed Mar. 19, 1971, and now abandoned, which is a divisionalof Ser. No. 37,610, filed May 15, 1970, and now U.S. Pat. No. 3,628,918,issued Dec. 21, 1971.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to the polymerization of ethylenealone or with comonomers or telogens (modifiers) at elevatedtemperatures and pressures in an elongated tubular reactor. Moreparticularly, the invention is directed to the production of solidpolyethylene under conditions wherein the bulk fluid velocity issufficiently high in the reaction zones of the tubular reactor so thatthe Flow Number in each reaction zone is greater than 3.3 ft.² /sec. Inits more specific aspects, the invention involves the production of highquality polyethylene under conditions wherein the bulk fluid velocity issufficiently high so that the Flow Number in each reaction zone in thetubular reactor having internal diameter between about 0.5 and 3 inchesis greater than 3.3 ft.² /sec. and the effective reaction volume isincreased to produce high quality polyethylene while controlling thepressure drop in the tubular reactor having at least two reaction zonesso as not to exceed 6,000 and, preferably, 3,000 psi at operatingpressures between 25,000 psi and 50,000 psi at the inlet of the tubularreactor as calculated or measured between the inlet of the firstreaction zone and the end of the last of the reaction zones.

2. The Prior Art

The polymerization of ethylene to solid polyethylene in an elongatedtubular reactor at elevated temperatures and pressures in the presenceof a free radical or free oxygen yielding initiator is known.Heretofore, however, the use of high bulk fluid velocities has beencarefully avoided due to the increased lengths of reaction zones thoughtto be necessary to accommodate the increased velocities in conventionaltubular reactors. It has been understood heretofore that to provide aconstant temperature rise per unit length of reaction zone in thetubular reactor, a doubling of the bulk fluid velocity would double thelength of reaction zone. Hence, increasing the bulk fluid velocity inthe reaction zone would necessarily increase the length thereofresulting in a much greater pressure drop occurring in the tubularreactor. In tubular reactors having one long or more than one shorterreaction zone, large pressure drops have detrimental effects on productoptical property and uniformity of other physical properties since thepolymer produced in the second or later portion of the reaction zone orzones are produced under lower pressures. The best film product isproduced at the highest pressure. Since for practical purposes, thereaction zone must be of a finite length and increasing the length ofthe reaction zone increases the pressure drop, the bulk fluid velocitieshave not exceeded about 36 ft./sec. in a one inch pipe; i.e., a FlowNumber of about 3.0.

SUMMARY OF THE INVENTION

The present invention may be briefly described as a process andapparatus for producing polyethylene in a tubular reactor having atleast one reaction zone at elevated temperature and pressure conditionsat bulk fluid velocities sufficient so that the Flow Number in eachreaction zone in the tubular reactor having internal diameters between0.5 and 3 inches is greater than 3.3 ft.² /sec. Significant to thepresent invention is the finding that at a Flow Number greater than 3.3ft.² /sec. the reaction zones having internal diameters between 0.5 and3 inches operate at the low end of the turbulent flow regime. Thisturbulent flow regime is characterized by a fully developed turbulentflow core, a buffer region and a laminar flow sublayer close to the tubewall. The flow regime was found by obtaining accurate pressure dropmeasurements which heretofore have not been obtainable with polyethylenepolymerization reactions. Another significant finding to the presentinvention is that the major portion of the reaction occurs in the fullydeveloped turbulent flow core of the turbulent flow regime. Stillanother significant aspect of the present invention is that pressuredrop may be controlled by use of side streams and/or use of largerdiameter cooling tubes making up the tubular reactor. More particularly,therefore, the present invention is directed to a process and apparatusfor producing polyethylene in a tubular reactor having at least onereaction zone at elevated temperature and pressure conditions at bulkfluid velocities sufficient so that the Flow Number in each reactionzone in the tubular reactor having internal diameters between 0.5 and 3inches is greater than 3.3 ft.² /sec. so that the effective reactionvolume is increased while controlling the pressure drop in the tubularreactor, wherein there are more than one reaction zone, between theinlet of the first reaction zone and the end of the last of the reactionzones so as not to exceed about 6,000 psi when operating at pressuresbetween 25,000 and 50,000 psi at the inlet of the first reaction zone.

VARIABLES OF THE INVENTION

The present invention is not limited to any specific tubular reactordesign, catalyst or initiator system or temperature or pressureconditions. Accordingly, these variables, while important to anyspecific process or apparatus, are set forth generally as background forthe the present invention.

The tubular reactor may be an elongated jacketed tube or pipe, usuallyin sections, of suitable strength and having an inside diameter betweenabout 0.5 to about 3 inches or more. The tubular reactor usually has alength to diameter ratio above about 100 to 1, and, preferably, having aratio from 500 to 1 to about 25,000 to 1.

The tubular reactor is operated at pressures from about 1,000 to about4,000 atmospheres. Pressures higher than 4,000 atmospheres may be used,but a preferred range is about 2,000 to about 3,500 atmospheres.

The temperatures employed are largely dependent on the specific catalystor initiator system used. Temperatures may range from about 300° toabout 650° F. or higher. The catalyst or initiator is a free radicalinitiator which may include oxygen, peroxidic compounds, such ashydrogen peroxide, decanoyl peroxide, diethyl peroxide, di-t-butylperoxide, butyryl peroxide, t-butyl-peroctoate, di-t-butyl peracetate,lauroyl peroxide, benzoyl peroxide, t-butyl peracetate, alkylhydroperoxides, azo compounds, such as azobisisobutyronitrile, alkalimetal persulfates, perborates, and percarbonates, and oximes, such asacetoxime to mention only a few. A single initiator or a mixture ofinitiators may be used.

The feedstock employed in the present invention may be ethylene orpredominately ethylene together with a telogen (modifier) or comonomer.Known telogens or modifiers, as the term is used herein, are illustratedby the saturated aliphatic aldehydes, such as formaldehyde, acetaldehydeand the like, the saturated aliphatic ketones, such as acetone, diethylketone, diamyl ketone, and the like, the saturated aliphatic alcohols,such as methanol, ethanol, propanol, and the like, paraffins orcycloparaffins such as pentane, hexane, cyclohexane, and the like,aromatic compounds such as toluene, diethylbenzene, xylene, and thelike, and other compounds which act as chain terminating agents such ascarbon tetrachloride, chloroform, etc. The process of the presentinvention may also be used to produce copolymers of ethylene with one ormore polymerizable ethylenically unsaturated monomers having a CH₂ ═C<group and which undergo addition polymerization. These copolymers may beproduced with or without modifiers present. Polymerizable ethylenicallyunsaturated monomers having a CH₂ ═C< group and which undergo additionpolymerization are, for example, alpha monoolefins, such as propylene,butenes, pentenes, etc., the acrylic, haloacrylic and methacryclicacids, esters, nitriles and amides, such as acrylic acid, chloroacryclicacid, methyacrylic acid, cyclohexyl methacrylate, methyl acrylate,acrylonitrile, acrylamide; the vinyl and vinylidene halides; the N-vinylamides; the vinyl carboxylates, such as vinyl acetate; the N-vinylaryls, such as styrene; the vinyl ethers, ketones or other compounds,such as vinyl pyridine, and the like. Comonomers and telogens ormodifiers are used to modify the properties of the ethylene polymerproduced. Accordingly, the term polyethylene, as used herein, is meantto include the so modified ethylene polymers as well ashomopolyethylene.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will be further illustrated by reference to thedrawings in which:

FIG. 1 is a flow diagram of a constant diameter tubular reactor havingtwo reaction zones for the polymerization of ethylene;

FIG. 2 is an illustration in cross section of the flow regime through areaction zone of a tubular reactor according to the present invention;

FIG. 3 is a flow diagram of a tubular reactor having two reaction zonesand a cooling zone of increased diameter; and

FIG. 4 is an illustration of a tubular reactor having three reactionzones according to this invention and provided with side streams anddifferent diameter tubes to provide high conversion, increased effectivereaction volume, i.e., reaction zone boundary layer control and pressuredrop control.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention may be illustrated by referring to FIG. 1 of thedrawings which illustrate a tubular reactor of constant diameter havingtwo reaction zones. Referring to FIG. 1, numeral 10 designates a feedline for introducing ethylene from a source (not shown) into a primarycompressor 11. The ethylene feed which may also include modifiers orcomonomers is then fed through line 12 to secondary compressor 13. Insecondary compressor 13, the feed gases are compressed for introductioninto the tubular reactor 14 at temperature and pressure conditions suchthat the gases are at supercritical conditions. Modifiers or comonomersmay be introduced into line 12 through line 15 and, in addition, recyclegases may be introduced into line 12 through line 16 before secondarycompressor 13. The compressors illustrated in FIG. 1 may be a singlecompressor or two or more in series or parallel. The side streams orrecycle streams may use separate compressors or lines from the primaryor secondary compressors.

The feed stream, after being introduced into tubular reactor 14, comesinto contact with an initiator introduced through line 17 whereby areaction occurs in reaction zone 18. The tubular reactor 14 is jacketedalong essentially its entire length with heat exchange media in thejacket to provide cooling for the highly exothermic reaction. After amaximum temperature is reached in the reaction zone, the reactionmixture cools in cooling zone 18C. Additional initiator may be addedthrough line 19 to provide a second reaction zone 20 wherein again amaximum temperature is reached followed by cooling in cooling zone 20C.The tubular reactor 14 is illustrated having two reaction zones;however, if additional reaction zones are desired, additional initiatormay be added by lines, such as line 21. At the end 22 of tubular reactor14, the reaction mass is discharged through a pressure let down valve 23into line 24 which leads to a high pressure separator 25.

In the high pressure separator 25, unreacted ethylene, modifiers and/orcomonomers are separated and recycled through line 16. The gases areappropriately cooled and wax separated (not shown) before recycled toline 12. The polymer is removed from the bottom of the high pressureseparator 25 through line 26 wherein it may pass through a second letdown valve 27 and through line 28 into a low pressure separator 29.

In low pressure separator 29, gases are removed through line 30. Thedesired polyethylene is removed through line 31 to be finished in aconventional manner.

In arriving at the present invention, a significant aspect was theability to measure the pressure drop in the tubular reactor 14. This wasaccomplished as illustrated in FIG. 1 by having a positive displacementpump 40 pump against the head at the end 22 of the tubular reactor 14.Hexane was pumped in very small amount, less than one gallon per hour,through line 42 having a check valve 43 therein. Instead of hexane, anyinert and/or compatible fluid with the polymer produced may be used. Thepositive displacement pump 40 having a pressure rating of about 60,000psi had a pressure gauge (not shown) thereon. As the hexane was pumpedthrough line 42, the pressure therein was the same as the pressure atthe end 22 of tubular reactor 14 and at any point in time was shown onthe pressure gauge as the head against which the pump 40 was pumping. Bythis technique, pressure measurements were obtained which heretoforewere unobtainable, because of difficulties such as plugging caused bythe presence of the polymer in attempting to use conventional pressuremeasuring devices, such as a gauge or strain gauge devices. As the letdown valve 22 opened under the normal pulsing or bumping cycle, hexanecontinued to flow in line 42 which prevents plugging therein andprovides accurate pressure measurements.

The following experimental data was obtained in a constant diametertubular reactor such as illustrated in FIG. 1. The results of theexperimental runs are set forth in Table I below. The experimental runswere at differing bulk flow velocities and a startling result was foundthat higher bulk fluid velocities than previously employed weredesirable to produce uniform, improved product. This was particularly sosince the additional reaction volume required at higher bulk fluidvelocities, normally considered in terms of additional length of tubularreactor, was obtained in reaction zone lengths essentially that used atmuch lower bulk fluid velocities. To express the present invention, acritical bulk flow velocity could not be defined since it would bemeaningful only for a specific diameter tube or pipe. In other words,the numerical value of the bulk fluid velocities which are necessary toobtain the results of the present invention would be different for eachdiameter pipe used in a tubular reactor. Accordingly, the term FlowNumber is used herein to express the nature of the flow regime in atubular reactor according to the present invention. Flow Number isdefined as the bulk fluid velocity in ft./sec. times the diameter infeet. In the following table, a comparison is given among runs atdifferent bulk flow velocities and at approximately the sameconversions, in a tubular reactor of one inch inside diameter.

                  TABLE I                                                         ______________________________________                                        V      FN       ΔP MI     Haze   Gloss                                  ______________________________________                                        35.7   3.0      1916     2.1    6.8    9.3                                    48.8   4.1      3332     2.4    5.2    10.4                                   64.1   5.3      5200     2.2    6.0    9.5                                    ______________________________________                                    

In the above table, V is bulk fluid velocity in ft./sec. FN is FlowNumber and MI is melt index. The pressure drop (ΔP) is that obtained bycalculation from the measured pressure drop between the inlet and outletof the tubular reactor for that portion of the tubular reactor betweenthe inlet to the reactor and the end of the last reaction zone. It isthis ΔP which is meaningful to produce quality since no further reactionoccurs and, accordingly, the pressure drop occurring in the last coolingzone is simply a matter of choice.

It can be seen from Table I that an improved film product is made at thehigher Flow Numbers in that the haze is lower and gloss higher. Thepressure drop is shown to have an effect on product quality since at thelarger pressure drops, the pressure in the second reaction zone is muchlower than in the first reaction zone and, accordingly, the polymerproduced therein is different.

It was found in analyzing the data from the experimental runs that theflow regime of the reaction mixture in the reaction zones of the tubularreactor was no longer in the higher portion of laminar flow, but in apartially turbulent flow regime. Hence, in calculating a friction factor(f) using the fundamental equations relating to flow of fluids in apipe, the friction factor was proportional to the Reynolds number to the-1/4 power. Significantly, this determination led to the presentinvention, wherein bulk fluid velocities should be increased in thepolymerization of polyethylene sufficiently to provide in the reactionzones of tubular reactors having internal diameters between 0.5 and 3inches, a Flow Number greater than 3.3 ft.² /sec. The nature of the flowregime in the reaction zones of the tubular reactor is illustrated inFIG. 2. The partially turbulent flow regime in reaction zone 18, forexample, comprises a fully turbulent flow core 45, a buffer region (notshown) and a laminar flow sublayer 46. The line 47 represents thevelocity profile and indicates that the velocity in the turbulent flowcore 45 is greater than in the laminar flow sublayer 46. Moresignificantly, however, now knowing the nature of the flow regime atthese bulk fluid velocities, it was concluded that even greater bulkfluid velocities for maximum turbulent volume were desired. Tosubstantiate this conclusion, the effective volume of the turbulent flowcore 45 and the percentage of the thickness of the laminar flow boundarylayer 46 were calculated. The results are shown in Table II using thedata obtained in the same runs set forth in Table I.

                  TABLE II                                                        ______________________________________                                               Effective Volume Laminar Thickness                                     FN     (% Total Volume) (% of Tube Diameter)                                  ______________________________________                                        3.0    52               29                                                    4.1    67               22                                                    5.3    74               18                                                    ______________________________________                                    

Concluded from the data were the following: that the major portion ofthe reaction occurs in the turbulent flow core 45, that the boundarylayer thickness or percentage of laminar thickness should be minimizedto increase effective reaction volume, that a more uniform product isproduced at increased effective volumes, that the bulk fluid velocitiesshould be increased over any presently used and sufficient to produce aFlow Number greater than 3.3 ft.² /sec., preferably greater than 3.5ft.² /sec., and suitably in the range of 4 to 10 ft.² /sec. for reactionzones in tubular reactors having diameters between 0.5 and 3 inches, andthat pressure drop should be controlled in tubular reactors having morethan one reaction zone to less than about 6,000 psi and, preferably,less than 4,000 psi at inlet pressures between 25,000 and 50,000 psisince large pressure drops between reaction zones affect product qualityand uniformity.

To illustrate the operability at very high Flow Numbers, an experimentalrun was made in a one inch internal diameter tubular reactor. Only onereaction zone was used due to limited heat transfer surface in thereactor. The results are set forth in Table III. Pressure drop at thisflow rate would have prohibited production of high quality film productif a second reaction zone were added.

                  TABLE III                                                       ______________________________________                                        V      FN       ΔP MI     Haze   Gloss                                  ______________________________________                                        91.5   7.6      9666     1.6    5.8    9.8                                    ______________________________________                                    

It may be concluded from the foregoing data in Tables I and III that aFlow Number greater than 3.3 ft.² /sec. produces an improved product,but that pressure drop must be controlled if a second reaction zone isused to obtain improved quality film product. Accordingly, one aspect ofthe present invention is to control the pressure drop in tubularreactors for producing polyethylene by using either side streaminjection or larger diameter cooling tubes, or both. This aspect of thepresent invention is illustrated with reference to FIG. 3, whereinethylene either alone or together with modifiers and/or comonomers isintroduced by line 50 into secondary compressor 51. The gases are thenintroduced by line 52 into a tubular reactor 53 having initial tubes ofa particular diameter. Initiator is introduced through line 54 into theinitial portion of the tubular reactor wherein reaction occurs in zone55. At that point in the tubular reactor 53, where the temperaturereaches a maximum, and cooling begins, tubes of a larger diameter areused to provide a cooling zone 56. In addition, if desired, coolingmedia may be introduced through side stream 57. The cooling media may beethylene, modifier, comonomer, an inert diluent or combinations thereof.After the reaction mixture passes through the cooling zone 56 of thetubular reactor 53, the mixture is introduced into a second reactionzone 58 wherein additional initiator is introduced by line 59, if notalready introduced in line 57. The reaction mixture is then passedthrough the tubular reactor 53 and through let down valve 60. Thereaction mixture is then separated in a separator 61 in a conventionalmanner with the gases being recycled through line 62 and the polymerproduct passing through line 63. According to the present invention,cooling tubes of larger diameter are used between reaction zones 55 and58 to provide a lower pressure drop between reaction zones because ofthe shorter length of tube required. Furthermore, a side stream 57 maybe provided to aid in cooling which will also reduce the length of thecooling zone required and the pressure drop between reaction zones. Byeither or both of these means, the bulk fluid velocities may beincreased over presently used velocities sufficiently to produce FlowNumbers greater than 3.3 ft.² /sec., preferably 3.5 ft.² /sec. or more,in each reaction zone having an internal diameter between 0.5 and 3inches so as to increase the effective reaction volume or the volume ofthe turbulent flow core while producing a higher quality product. Bycontrolling the pressure drop to less than 6,000 psi, preferably lessthan 4,000 psi, between the inlet of the tubular reactor having apressure between 25,000 and 50,000 psi and the end of the last reactionzone in the tubular reactor, high quality film product may be produced.

To illustrate the present invention in all of its aspects, a tubularreactor for the production of polyethylene is illustrated in FIG. 4. Thefeed gases are introduced by line 70 to a preheater 71 to be heatedbefore introduction into the jacketed tubular reactor 72. The tubularreactor 72 has six zones including three reaction zones and threecooling zones. Initiator is introduced into reaction zones 1, 3, and 5,while side streams for cooling and reducing pressure drop are introducedinto cooling zones 2 and 4. The following Table IV sets forth theinternal diameter of the tubes in each zone, the bulk fluid velocitiesin each zone and the Flow Number in each reaction zone for a particulardesign. In any particular design, the first reaction zone may have aninternal diameter between 0.5 and 2.5 inches.

                  TABLE IV                                                        ______________________________________                                               Internal  Bulk Fluid                                                          Diameter  Velocity                                                            (Inches)  (Ft./Sec.)   FN                                              ______________________________________                                        Zone 1   1           52.5         4.37                                        Zone 2   1           52.5         --                                          1.25     56.4        --                                                       Zone 3   1.25        56.4         5.87                                        Zone 4   1.50        39.2         --                                          Zone 5   1.25        79.0         8.23                                        Zone 6   1.50        55.0         --                                          ______________________________________                                    

The pressure drop in the tubular reactor 72 having the foregoingdimensions would be less than 2500 psi between the inlet to zone 1 andthe end of zone 5 at inlet pressures in excess of 35,000 psi. The twodifferent diameters for the first cooling zone 2 illustrates thatmaintaining the same internal diameter tubes for a certain length toprovide cooling before increasing the internal diameter is within thepresent invention and is a matter of design. For example, all of coolingzone 2 may be 1.25 inches internal diameter. However, it is desirable tointroduce the side stream to the larger diameter tube. The length of thecooling zone in the larger diameter tube may be very short (less than 50ft.) before another reaction is initiated by introduction of initiator.While various design changes may be made, according to the presentinvention, the important feature is that the bulk fluid velocities ineach reaction zone are sufficient to provide a Flow Number greater than3.3 ft.hu 2/sec. Further, it is desirable to have larger Flow Numbers ineach successive reaction zone so as to provide greater effectivereaction volumes to offset the effect of having polymer in the reactionmass in the subsequent reaction zone. With the tubular reactor 72,illustrated with the foregoing dimensions, the effective reaction volumeis 64% in zone 1, 72% in zone 3, and 84% in zone 5. Since essentially noreaction is occurring in the cooling zones 2, 4, and 6, the flow regimeis not critical to the kind of product produced and, accordingly, muchlower bulk fluid velocities in the larger diameter cooling tubes may beused. The larger diameter tubes provides additional heat transfersurface as well as reduces overall pressure drop.

In summary, the present invention is directed to a process for producingpolyethylene in reaction zones having internal diameters between 0.5 and3 inches at bulk fluid velocities sufficient so that the Flow Number isgreater than 3.3 ft.² /sec. in each reaction zone. Preferably, tubularreactors having at least two reaction zones or more are used and theFlow Number in each successive reaction zone will be equal to or,preferably, higher than the reaction zone it follows. Further, intubular reactors having at least two reaction zones, the pressure dropbetween the inlet of the tubular reactor and the end of the lastreaction zone is controlled to less than 6000 psi, preferably 4,000 psiwhen the inlet pressures are between 25,000 and 50,000 psi. The controlof pressure drop is accomplished by side stream injection and/or largerdiameter tubes in the cooling zones.

The nature and object of the present invention having been completelydescribed and illustrated and the best mode contemplated set forth, whatwe wish to claim as new and useful and secure by Letters Patent is: 1.In a process for producing polyethylene in a tubular reactor having aninlet and an outlet and at least one reaction zone having an internaldiameter between 0.5 and 3 inches, at a temperature within the rangebetween about 300° F. and about 650° F. and at a pressure between about1,000 and 3,500 atmospheres and in the presence of a free radicalyielding initiator, the improvement which comprises:passing the reactionmass through each reaction zone in said tubular reactor at a bulk fluidvelocity sufficient so that the Flow Number is greater than 3.3 ft.²/sec. and wherein the total pressure drop within said tubular reactor iscontrolled to be less than 6,000 psi.
 2. A process according to claim 1wherein said Flow Number is greater than 3.5 ft.² /sec.
 3. A processaccording to claim 1 wherein the Flow Number is within the range betweenabout 4.0 to 10 ft.² /sec.
 4. A process according to claim 2 whereinthere are at least two reaction zones.
 5. A process according to claim 4wherein the Flow Number is successively higher in each reaction zone. 6.A process according to claim 1 wherein the pressure drop is controlledto less than 4,000 psi.
 7. A process according to claim 1 wherein saidpressure drop control between said inlet of said tubular reactor and theend of said last reaction zone controlled to be less than 6,000 psi isaccomplished by a procedure comprising one or more of the followingsteps:(a) expanding the volume occupied by said reaction mass; (b)introducing a cool side stream into said reaction mass; or (c) acombination of steps (a) and (b).
 8. A process for producingpolyethylene in an elongated tubular reactor having at least tworeaction zones and an inlet and an outlet, at a temperature within therange of about 300° to about 650° F., and at a pressure between about1,000 and 3,500 atmospheres, and in the presence of a free radicalyielding initiator which comprises:(a) passing a reaction masscomprising ethylene and initiator through a first reaction zone havingan internal diameter between about 0.5 and 2.5 inches in said tubularreactor at a bulk fluid velocity sufficient so that the Flow Number isgreater than 3.3 ft.² /sec., (b) subsequently passing said reaction massthrough a cooling zone having an internal diameter greater than thediameter of said first reaction zone, and cooling said reaction masswithin said said cooling zone to no lower than a temperature of about300° F., (c) passing said cooled reaction mass through one or moreadditional reaction zones each reaction zone being separated by acooling zone said reaction zones having an internal diameter no greaterthan about 3 inches but smaller than or equal to the internal diameterof the preceding cooling zone at a bulk fluid velocity sufficient sothat the Flow Numbers in the additional reaction zones are at least thatin said first reaction zone, (d) controlling the pressure drop frominlet to outlet to be less than 6,000 psi.
 9. A process according toclaim 8 wherein the internal diameter of said first reaction zone isbetween 0.5 and 2 inches and said Flow Number is greater than 3.5 ft.²/sec.
 10. A process according to claim 9 wherein an ethylene side streamis injected into said cooling zone.
 11. A process according to claim 8wherein the internal diameter of the first of said additional reactionzones is equal to the internal diameter of said cooling zone.
 12. Aprocess according to claim 8 wherein the internal diameter of the firstof said additional reaction zones is less than the internal diameter ofsaid cooling zone.
 13. A process according to claim 8 wherein thepressure drop between said inlet of said tubular reactor and the end ofthe last reaction zone is controlled to be less than 4,000 psi.
 14. Aprocess for producing high pressure polyethylene which comprises incombination the steps of:(a) providing a series of tubular reaction zonesegments having internal diameters of between about 0.5 to less than 3inches interconnected with cooling zone segments whereby each coolingzone segment follows and is associated with a reactor zone segmentexcept for the last of said reaction zones; (b) reacting a fluidreaction mixture of ethylene and newly introduced free radical initiatorin each of said reaction zones at a temperature of from about 300° F. toabout 650° F. and at a pressure of from 1,000 to 4,000 atmospheres; (c)maintaining the bulk fluid velocity of said reaction mixture at a FlowNumber of 3.3 and above within each of said reaction zones; and (d)conveying the reaction mixture from a reaction zone into the nextcooling zone and avoiding excessive pressure drop of more than 6,000 psiby a procedure selected from:(i) causing expansion of the volumeoccupied by said reaction mixture in said cooling zone; (ii) introducingcooling media into said cooling zone; or (iii) a combination of (i) and(ii).
 15. A process according to claim 14 wherein said excessivepressure drop is avoided by said expansion procedure alone.
 16. Aprocess according to claim 14 wherein said excessive pressure drop isavoided by said cooling procedure alone.
 17. A method according to claim14 wherein said expansion is accomplished by passing said reactionmixture into said cooling zone which has an internal diameter greaterthan that of the immediately preceding reaction zones, and greater thanthat of the immediately subsequent reaction zone.
 18. A processaccording to claim 14 wherein the Flow Number in each cooling zonesegment is from 20 to 75% less than that of the Flow Number in thepreceding reaction zone, and wherein the bulk velocity of said reactionmixture in said cooling zone segment is different from the bulk fluidvelocity of said reaction mixture in said reaction zone segment.
 19. Aprocess according to claim 14 wherein the Flow Number in each successivereaction zone is higher than that in the preceding reaction zone.
 20. Aprocess according to claim 14 wherein the internal diameter of eachsubsequent reaction zone is less than the internal diameter of saidpreceding cooling zone and wherein ethylene without initiator isintroduced into said cooling zone and initiator is separately introducedinto the initial portion of at least one of said reactor zones.
 21. Aprocess according to claim 14 wherein said cooling media is ethylene.